1. Field of the Invention
The present invention relates to controlling a bias current value for each of a plurality of light emitting elements in an electrophotographic image forming apparatus which forms an image on photosensitive members by using light beams emitted from the plurality of light emitting elements.
2. Description of the Related Art
An electrophotographic image forming apparatus such as a laser beam printer forms electrostatic latent images by scanning the surfaces of photosensitive members such as photosensitive drums by using laser beams emitted from a semiconductor laser and then developing the electrostatic latent images by using toner. With such an image forming apparatus, it is necessary to increase the ON/OFF switching speed of the semiconductor laser to deal with the increase in image resolution and operating speed in recent years. FIG. 14 illustrates a waveform (light intensity waveform) representing the intensity of a laser beam when the semiconductor laser is switched from the OFF state to the ON state. Referring to FIG. 14, the horizontal axis is assigned time. The solid line represents change in drive current supplied to a relevant light emitting element with time, and the dotted line represents change in light intensity (light amount) with time. Ideally, it is desirable that the waveform of the drive current (hereinafter referred to as drive current waveform) is identical or similar in shape to the light intensity waveform. However, as illustrated in FIG. 14, even when the drive current is supplied to the semiconductor laser, the light intensity waveform does not rise for a certain fixed period of time. Hereinafter, this phenomenon is referred to as degradation in light emission response of the semiconductor laser.
A semiconductor laser has the light emission characteristics as illustrated in FIG. 15A. Referring to FIG. 15A, the horizontal axis is assigned a drive current supplied to the semiconductor laser and the vertical axis is assigned a light emission amount (light intensity) of a laser beam corresponding to the supplied drive current value. As illustrated in FIG. 15A, the light emission amount increases slowly with respect to the increase in drive current value in a region where the drive current value supplied to the light emitting element is lower than a threshold current value Ith, and increases steeply with respect to the increase in drive current value in a region where the drive current value is higher than the threshold current value Ith.
To restrain the above-mentioned degradation in light emission response, the semiconductor laser is turned ON by supplying a bias current Ib instead of supplying a drive current from the OFF state in which no drive current is supplied to the semiconductor laser. The bias current Ib is set to such a value as to emit a laser beam having such a light amount that does not change the surface potential of a photosensitive member. When changing the surface potential of the photosensitive member, a drive current composed of the bias current Ib and a switching current Isw superimposed thereon is supplied to the semiconductor laser. Then, the semiconductor laser emits a laser beam having such an intensity that changes the surface potential of the photosensitive member. On the other hand, in a light emission wait state, only the bias current Ib is supplied to the semiconductor laser. Although the semiconductor laser enters a weak light emission state when the bias current Ib is supplied, the laser beam emitted from the semiconductor laser only by the bias current Ib has a low intensity and, therefore, the surface potential of the photosensitive member remains unchanged. Applying the bias current Ib to the semiconductor laser in a period for forming an electrostatic latent image on the photosensitive member in this way enables restraining the degradation in light emission response (light emission delay) when the switching current Isw is supplied to the semiconductor laser.
To restrain the degradation in light emission response, it is desirable to set the bias current value Ib to a value as close as possible to the drive current value for emitting a laser beam having such an intensity that changes the potential on the photosensitive member.
Japanese Patent Application Laid-Open No. 11-245444 discusses the following technique as a conventional method for setting the bias current Ib with sufficient accuracy. In automatic power control (hereinafter referred to as APC) for determining a drive current that achieves a constant light amount of laser beam, as illustrated in FIG. 15A, drive currents I1 and I2 are measured. The drive current I1 is a drive current necessary for light emission with a first light amount P1. The drive current I2 is a drive current necessary for light emission with a second light amount P2 which is lower than the first light amount P1 (for example, one fourth thereof) as a target value. The light amount of laser beam is measured by a photodiode (PD), and the drive current value supplied to the semiconductor laser at the time of image formation is controlled based on a result of light amount detection by the PD. The PD is disposed in the vicinity of the light emitting element at such a position where the PD receives the laser beam (rear beam) emitted in a direction opposite to the direction of the laser beam (front beam) toward the photosensitive member. When the semiconductor laser emits the front beam, it also emits the rear beam in response to the front beam emission. The intensity of the front beam has a relation (for example, a proportionality relation) with the intensity of the rear beam.
Referring to the graph in FIG. 15A, which represents a relation between the drive current and the light amount (light emission characteristics), a straight line connecting a point defined by the light amount P1 and the drive current I1 and a point defined by the light amount P2 and the drive current I2 is obtained. Then, an intersection of a line segment extending from the straight line and the horizontal axis (light amount zero) is obtained, and the current value for the intersection is set as the threshold current value Ith. Although the actual threshold current value Ith is a current value at which the inclination of the light emission characteristics changes in FIG. 15A, processing for grasping the light emission characteristics in detail is required to calculate the actual threshold current value Ith. To obtain the threshold current value Ith, it is necessary to turn ON the semiconductor laser by using at least three different light amounts, calculate these light amounts and current values corresponding thereto to obtain the light emission characteristics, and set the threshold current value Ith based on the light emission characteristics. However, this method takes much control time to obtain the threshold current value Ith.
Japanese Patent Application Laid-Open No. 11-245444 discusses a laser diode drive apparatus which sets a current value obtained by the above-mentioned method as the threshold current value Ith. The laser diode drive apparatus utilizes the fact that, when a high current value is supplied to the semiconductor laser, the light emission amount linearly changes with varying current value. The threshold current value Ith is multiplied by a predetermined coefficient α, or a predetermined correction value is subtracted from the threshold current value Ith or added to the threshold current value Ith in order to obtain the bias current Ib. Setting the bias current Ib in this way enables preventing the emission of a laser beam having such an intensity that changes the potential on the photosensitive member from the semiconductor laser when only the bias current Ib is supplied.
Light emission with the first light amount P1 and light emission with the second light amount P2 are performed for every other scanning in this way. Thus, even when the threshold current value Ith varies by temperature change in the light emitting element, the bias current Ib can be set in relation to the variation in the threshold current value Ith.
However, in an image forming apparatus which exposes a photosensitive member to a plurality of laser beams emitted from a plurality of light emitting elements, detecting laser beams (rear beams) emitted from a plurality of light emitting elements by using one PD and performing APC based on a result of light amount detection intending to improve the image forming speed causes a problem that the bias current Ib cannot be set with high precision.
When performing APC, a drive current necessary for light emission with the first light amount P1 and a drive current necessary for light emission with the second light amount P2 are supplied to the light emitting element under control, and the bias current Ib corresponding to the light emitting element under control is calculated based on the above-mentioned conventional method. This control is sequentially performed during one scan for each of the plurality of light emitting elements.
In this case, the bias current Ib is supplied to light emitting elements other than the one under control to ensure proper light emission response. The bias current Ib is set before each scanning. Since the plurality of light emitting elements is disposed in the vicinity of the PD, the PD receives the laser beam emitted only by the bias current Ib. Therefore, the result of light amount detection by the PD includes the light amount emitting elements other than the one under control.
In the process for calculating the bias current value Ib based on the conventional method with such an image forming apparatus, a drive current I1′ corresponding to the first light amount P1 and a drive current I2′ corresponding to the second light amount P2 are calculated (refer to FIG. 15B). As a result of this calculation, as illustrated in FIG. 15B, a calculated threshold current value Ith′ is lower than the proper threshold current value Ith, and accordingly the bias current value Ib is set to a value lower than the proper current value. With an image forming apparatus which forms an electrostatic latent image by using a plurality of light emitting elements, the bias current value Ib is set to a value remarkably lower than the threshold current value Ith in this way. This causes the degradation in the semiconductor laser response when the switching current Isw is supplied.
One of the possible solutions for this problem is to correct the calculated bias current value Ib so that it comes close to the threshold current value Ith. This correction is achieved by adding a correction value to the bias current value Ib or multiplying the bias current value Ib by a coefficient equal to or greater than one. However, the sensitivity (the ease with which the surface potential changes) of the photosensitive member fluctuates by a temperature or humidity change as well as the aging of a photosensitive layer of the photosensitive member. Therefore, when the bias current value Ib is corrected based on a fixed parameter (a correction value or coefficient), a latent image may be formed on the photosensitive member by a laser beam emitted from a light emitting element to which the corrected bias current value Ib is supplied.